Recently, in line with realization of an optical amplifier (EDFA: Erbium Doped Fiber Amplifier) using an optical fiber to which erbium is doped, it became possible to directly amplify optical signals in a wavelength band of 1.55 .mu.m (1550 nm) without converting them to electric signals, whereby in a field of optical transmission, high bit rate and long-haul optical transmission has been achieved. Furthermore, on the other hand, transmission is carried out by a wavelength division multiplex system (WDM system) in which optical signals of different wavelengths can be transmitted by a single optical fiber, in order to increase transmission capacity in optical transmission, wherein, by employing an optical amplifier using said erbium doped optical fiber in an optical transmission system in which the wavelength division multiplex system, it can be expected that the transmission capacity is further increased, and a longer haul transmission is further achieved by the wavelength division multiplex system.
Also, as an optical fiber used for a wavelength division multiplex transmission system in which said EDFA is employed, a dispersion shifted optical fiber having zero dispersion wavelength in the wavelength band of 1550 nm has been proposed. However, in a case where an optical fiber has a zero dispersion wavelength in almost the entire gain band of the said EDFA, it is already known that a four-wave mixing (FWM) which is one of the non-linear phenomena occurs when light of a plurality of wavelengths is made incident into the optical fiber. Signal light power is lost due to the four-wave mixing, wherein the original signals are converted to signals of other wavelengths and become noise, resulting in a problem in the transmission characteristics. Furthermore, any one of the conventional dispersion shifted optical fibers has a so-called positive dispersion slope by which the dispersion value (chromatic dispersion value) is increased in line with an increase in wavelength, and the positive dispersion slope by which the dispersion value is generally about 0.07 ps/nm.sup.2 /km.
Therefore, in order to eliminate the problem resulting from the four-wave mixing, an attempt, by which said problem resulting from the four-wave mixing is solved, has been made by separating the dispersion value at the wavelength used and setting the dispersion value at the wavelength used, so that it becomes .+-.1.5 ps/nm/km to .+-.4 ps/nm/km. This is described in Japanese Laid-Open Patent Publication No. 168046 of 1995. An optical fiber which is able to solve the four-wave mixing is registered by the AT & T Corporation with the trademark "TRUE WAVE". Furthermore, the TRUE WAVE is based on the NEO DSF concept (Dispersion shifted optical fiber of the next generation). An optical fiber called TRUE WAVE has a zero dispersion wavelength in the vicinity of wavelength of 1520 nm or in the vicinity of wavelength 1580 nm and has a positive dispersion slope.
Hereinafter, like the TRUE WAVE and the said conventional general dispersion shifted optical fibers, an optical fiber having a positive dispersion slope is called "Positive dispersion slope optical fiber".
Since the dispersion of the positive dispersion slope optical fiber called TRUE WAVE is not zero at the wavelength used as described above, dispersion resulting from the TRUE WAVE itself will occur at the use wavelength band. Therefore, in a case where the TRUE WAVE is used, the following measures are taken. That is, TRUE WAVE having the chromatic dispersion characteristics shown by the characteristics line b1 in FIG. 7 and TRUE WAVE having the chromatic dispersion characteristics shown by the characteristics line b2 in the same drawing are alternately connected to each other, whereby the TRUE WAVE is caused to have, as a whole, chromatic dispersion characteristics shown by the characteristic line b3 in the same drawing, and an attempt is made by which the dispersion at the center wavelength (for example, 1550 nm) of the wavelength is made zero.
However, if so, the dispersion at the center wavelength becomes zero, residual dispersion occurs at wavelengths other than the wavelength, for example, residual dispersion of -1.6 ps/nm/km is produced at a wavelength of 1540 nm, wherein if the relay interval (transmission distance) is 80 Km, dispersion of 128 ps/nm remains. In this case, a difference is produced in the transmission capacity due to dispersion between various wavelengths of signal light at the wavelength band of 1550 nm, the waveform of the signal light is distorted to cause the bit error rate to be changed between the center wave and the end waves, wherein the efficiency of the wavelength division multiplex transmission is lowered and reliability may be remarkably lowered.
Therefore, in Japanese Patent Application No. 85846 of 1997 filed at the Japanese Patent Office, the present applicant proposed a dispersion slope compensating optical fiber in which the dispersion slope of a positive dispersion slope optical fiber of, for example, said TRUE WAVE, etc., is compensated. Furthermore, the dispersion slope compensating optical fiber is an optical fiber which has a dispersion value of a general dispersion shifted optical fiber which is a positive dispersion slope optical fiber, and a negative signed dispersion value, the absolute value of which is equivalent thereto, and has a negative signed dispersion slope, the absolute value of which is equal to that of the dispersion shifted optical fiber.
However, in order to compensate the positive dispersion slope of TRUE WAVE by, for example, connecting the dispersion slope compensating optical fiber to the existing line (optical transmission line) composed of the TRUE WAVE, it is necessary to connect the dispersion slope compensating optical fiber, the length of which is almost equal to that of the TRUE WAVE of the existing line. Therefore, it will become necessary to provide a very long dispersion slope compensating optical fiber. For this reason, it is difficult to insert, as a module, the proposed dispersion slope compensating optical fiber into, for example, a repeater of the existing line and to use it therein.
Furthermore, on the other hand, since the gain band of EDFA is liable to expand to a longer wavelength side, it will become possible to amplify optical signals in the vicinity of a wavelength of 1600 nm. Therefore, the necessity of dispersion slope compensating optical fiber used in this wavelength bandwidth has been increased.
The present invention is developed in order to solve the said problem, and it is therefore an object of the invention to provide a dispersion slope compensating optical fiber which, in the wavelength band, from 1530 nm to 1610 nm in wavelength, which is the gain band of EDFA, is able to almost compensate the chromatic dispersion slope and chromatic dispersion of a positive dispersion slope compensating optical fiber such as TRUE WAVE, etc., by a short optical fiber, and for example, by inserting it into the existing line having a positive dispersion slope for optical transmission as a module, and enables satisfactory wavelength division multiplex transmission at the said wavelength band.